Logging tools are lowered into a well on a logging cable. The cable typically includes one or more electrical conductors extending from the logging tool in the sonde (one or more tools) to the end of the cable which is typically stored on a drum or reel for convenience in handling. The logging cable typically has a strong member which is a type of woven wire rope. The length of the cable is an important factor to enable depths or locations of measurements to be determined by the sonde in the borehole. Thus, a particular reading must be located vertically so that the reading can be correlated with other data to associate the data with particular formations which are intercepted by the borehole. One technique for increasing the precision of this measurement is to place magnetic marks on the cable at specified locations. Magnetic marks typically are placed on the cable by magnetizing the ferrous material comprising the cable. Ordinarily, the magnetic marks are placed at calibrated distances along the cable.
A magnetic mark is placed by magnetizing that portion of the cable to create a magnetic mark. The mark is detected by a detection device which is held adjacent to the cable as it moves past which forms an output pulse from the device. The pulse is indicative of the passage of the magnetic mark. This works quite well as long as the magnetic mark has a specified magnetic intensity or strength. However, the magnetic marks tend to wear by loss of intensity. Several factors contribute to the loss of intensity. One is the mere passage of time, but perhaps the more important factors involved are continual passage of the marked portion of the logging cable over the sheave. Another factor is prior magnetization of the casing or wellhead equipment, whether intentional or accidental. In addition, the magnetic marks on the cable are stored in tightly wound bights on the reel, and marks stored adjacent to one another tend to reduce mark intensity. Accordingly, the marks on the cable become weaker and weaker. Ultimately, they become so weak that they can no longer be detected, or at least several marks of the cable become so weak that the entire cable must be removed and remarked. Remarking is easily described but is an expensive process to carry out. It is quite difficult to accomplish in the field. Normally, it requires that the cable be removed from the field location (whether onshore or offshore) and transferred with the storage reel to a remarking location. The remarking equipment requires a mechanism for magnetizing the cable. In addition, it requires a measuring apparatus which carefully calibrates the distance between adjacent marks. The strong marks are placed on the cable only after precise measurement. The first step, however, is erasing all prior marks. This is accomplished simply by exposing the entire cable to an oscillatory magnetic field which destroys the preexistent marks. Remeasuring then positions the new set of marks. The new marks are simply placed on the cable in the same fashion as would occur with a new cable.
The remarking procedure presently used thus requires transportation of the cable and associated reel, thereby taking the cable out of service. This is disruptive to field operations, and is a time consuming process which is therefore relatively expensive. In particular, this derives from the necessity of remeasuring the cable to locate the marks. This must be accomplished with the cable held at a specified tension or load. The materials comprising the cable stretch or elongate in the elastic range. Accordingly, the marks must be placed on the cable with a specified or known tension. Accordingly, the presently used remarking procedure involves precise measurements of cable tension and distance between marks. It is desirable that the marks be placed on the cable with an accuracy which approaches about one eighth inch between marks which are normally one hundred feet apart. In other words, the measurement is accurate to approximately one part per ten thousand between marks. While this accuracy can be obtained, it is nevertheless obtained with substantial time, effort and resultant cost.
By contrast, the present apparatus and procedure remark the cable by intensifying the prior marks. The prior marks will fade, but they are remarked before they become undetectable. Thus, when the cable has been used sufficiently that the marks are marginally detectable, they are remarked at the same locations. This avoids the necessity of careful measurements. This avoids the necessity of placing the cable under a specified tension during remarking. This is more readily accomplished with equipment in the field, and indeed, the present apparatus has the form of an accessory which can be incorporated with the cable handling equipment so that the cable and drum for storage of the cable do not have to be removed; rather, remarking can occur at the wellhead. It can occur while leaving the cable drum mounted on the skid or other support equipment without any disruption of the equipment and removal from the immediate wellhead vicinity.
Remarking as proposed by the present disclosure involves the use of a set of measuring wheels which are already in place. The measuring wheels enable the cable to be spooled into the borehole while the magnetic marks pass through a detector. The detector forms an output signal having the form of an electrical pulse indicative of magnetic mark passage. That signal is delivered to a marker control circuit. That circuit provides a strong magnetization current flow to a coil positioned around the cable for marking purposes. The magnetic mark formed thereby is controlled in location so that it marks over the prior mark. In other words, as the prior mark traverses through the present apparatus, it is written over, thereby providing a strong mark on the cable using prior mark locations.
The present procedure involves the use of a magnetic mark detector and a magnetic marker. The latter device is a coil provided with a pulsed current flow of sufficient strength. The distance between these two components is measured along the cable passing through and is a distance which can be measured. As the cable traverses the equipment, this distance is also traversed by a mark which is to be remarked, and this distance is measured by the controller circuitry. Remarking is then implemented. Measurements between marks are not required. Calibrated measurements under calibrated tension loads are not required. Rather, the entire procedure is accomplished in the field utilizing the prior marks to locate the new and stronger marks.